The present invention relates to an optical transmission device and an optical transmission system. More particularly, the present invention relates to an optical transmission device and an optical transmission system suitable for low-noise transmission.
In an attempt to satisfy a requirement of lowering the cost for an optical transmission system, a wavelength divisional multiplexing optical transmission system, which transmits different wavelengths of signal lights in one single optical transmission fiber, has been considered. In particular, a bi-directional optical transmission system, which transmits different wavelengths of light signals in a single optical transmission fiber bi-directionally, is suitable when exchange of information is needed interactively between the two connected stations. Under such a technical background, it has become more important to provide an optical amplifier applicable to a bi-directional optical transmission system.
Japanese Patent Laid-open No. Hei 6-85369 describes as a conventional apparatus an optical amplifier. The optical amplifier includes apparatus for multiplexing or demultiplexing signal lights in a forward or a reverse direction toward both ends of a doped fiber. The optical amplifier is capable of sharing the use of one optical amplifying medium and one optical pumping source in the forward or the reverse direction, and is applicable to a bi-directional optical transmission system, the constitution of which is simple.
Japanese Patent Laid-open No. Hei 9-98136 describes another example of an optical amplifier which is capable of controlling the individual wavelength output even if there occur variations in signal wavelength multiplexity.
The optical amplifiers disclosed by the above-identified Japanese patent applications have various disadvantages in their practical use as described below. It is generally known that, in a one-directional optical amplifier having one doped fiber, a signal light input loss at a step previous to the doped fiber is attributed to a degradation in the S/N ratio.
xe2x80x9cOptical Amplifiers and Their Applicationxe2x80x9d (Ohm Publishing, May, 1992, pp 5-3[1]), describes that it is essential to combine an optical isolator at the front of doped fiber for suppressing reflexed amplified spontaneous emission (ASE). The optical isolator is not the only optical component which is inserted at the front of doped fiber. Generally, a transmission equipment requires a wavelength demultiplexer for an optical surveillance signal, a optical coupler for an optical signal monitor and a wavelength multiplexer for a pumping light. All of these optical components have loses. Further, the noise figure of Erbium doped fiber having a length of 20-30 m is not negligible. Where the noise figure is defined by the ratio of the S/N ratio on the input side and the S/N ratio on the output side.
The optical signal which is attenuated in the transmission path also suffers losses due to the optical components. The optical signal is amplified in the EDF of which a noise figure is large. The above-described transmission equipment cannot achieve a noise figure less than 6 dB.
When a non-regenerative multiple amplifying transmission is performed using k units of optical amplifiers, the S/N ratio degradation amount increases in proportion to the step number k. Accordingly, in an actual optical transmission system in which there exists an upper limit in the total S/N ratio degradation amount, the repeating step number decreases as the S/N ratio degradation amount in the optical amplifiers increases. This eventually shortens the light transmission distance.
For example, when setting optical amplifiers, the S/N ratio degradation amount of same are 4 dB, and the S/N ratio degradation amount of others are 6 dB at intervals of 80 km. Under a requirement that the total S/N ratio deterioration amount can not be more than 12 dB, a total S/N ratio degradation amount of the 4 dB optical amplifiers becomes 12 dB when three steps are repeated, and the total S/N ratio degradation amount of the 6 dB optical amplifiers becomes 12 dB when two steps are repeated. Thus, when the 4 dB optical amplifiers are used in three repeated steps it is possible, thus making it possible to transmit a signal light for 240 km. Whereas, when the 6 dB optical amplifiers are used in two repeated steps it is possible to transmit a signal light for 160 km.
A first object of the present invention is to eliminate the above-described inconvenience as well as to provide an optical transmission device which is applicable to the low-noise optical transmission system and is, suppressing a degradation of the S/N ratio, suitable for a long haul optical transmission.
A second object of the present invention is to provide a bi-directional optical transmission system suitable for the long distance optical transmission.
In order to solve the above-mentioned problems, a terminal station repeater or an in-line repeater is configured by at least one buffer light amplifying unit in contact with a transmission path and at least one core light amplifying unit in contact with the buffer light amplifying unit. This configuration allows the buffer light amplifying unit to amplify an input signal before a signal light, which has been attenuated because of the propagation along the transmission path, suffers from losses from the optical devices, thereby making it possible to prevent noise degradation in the optical transmission device.
By use of the present invention it is possible to embody an optical transmission device in an optical transmission system, wherein degradation of the S/N ratio is suppressed. Thus, the present invention is suitable for long haul optical transmission. Further, by employing the optical transmission device of the present invention it is possible to develop an optical transmission system suitable for the long distance optical transmission.
The present invention provides an optical transmission device which reduces optical noise in bi-directional transmission systems. The optical transmission device includes a core light amplifying unit and a first buffer light amplifying unit for amplifying a first signal light from a first transmission path and an amplified second signal light from the core light amplifying unit. The first buffer light amplifying unit supplies the core light amplifying unit with the first signal light, and supplies the first transmission path with the amplified second signal light. A second buffer light amplifying unit is provided for amplifying a second signal light from a second transmission path and an amplified first signal light from the core light amplifying unit. The second buffer light amplifying unit supplies the core light amplifying unit with the second signal light, and supplies the second transmission path with the amplified first signal light.
The core light amplifying unit includes a first optical multiplexer/demultiplexer, a second optical multiplexer/demultiplexer, a first optical amplifier for amplifying the first signal light from the first optical multiplexer/demultiplexer so as to send out the amplified first signal light to the second optical multiplexer/demultiplexer, and a second optical amplifier for amplifying the second signal light from the second optical multiplexer/demultiplexer so as to send out the amplified second signal light to the first optical multiplexer/demultiplexer.